I am curious about the notion of a plain (plane?) nose bearing allowing high-speed running. Typically, plain bearings can limit safe high speed running, although 4000 RPM is not that high as spindle speeds go. It's quite high for a lathe spindle, but milling spindles running on very low tolerance ball or roller races are turning ten times that in some special machines. Is the implication that if you had a ball or roller bearing headstock you could not turn that fast?
I use both and prefer carbide for harder stuff as others have said. While carbide does offer the ability to do high-speed machining, that is rarely important to folks using machines in the Super 7 class (home shop types). Usually we are looking for better surface finish and that often comes from the use of HSS. The other big advantage to HSS is that its easy to tweak cutter geometry to improve finish....something that is more limited with insert cutters that require the purchase of a package of fairly standard inserts.
People often labor over grinding a HSS tool. While care in desiging the angles and grinding them is a good idea, resharpening does not need to require regrinding. Often a few strokes of a good stone will bring an edge back up to sharpness. "Sharp" carbide inserts for insert holders are often pretty rare and that can make for more load on a machine as well as for the need for deeper depths of cut. As a home shop type, I often sneak up on a dimension and my last cut may be a few thousandths...which is often less than the ideal depth or chip load for a carbide insert with its chip breaker back a good ways from its leading edge (by design).